The EPC process entails the use of inexpensive and expendable patterns that are made from a low melting point, low density, highly vaporizable material such as expanded polystyrene. The pattern is coated with a refractory wash that is typically comprised of fine silica in an organic vehicle/binder. The wash is dried, and the coated pattern is placed in a flask to which unbonded sand is added. The sand is compacted and densified around the pattern by means of fluidization and vibration. Molten metal is poured directly into the pattern, which immediately vaporizes momentarily leaving a cavity. The metal takes the place of the pattern in its exact configuration. Gases from the pattern exit through the sand.
Expanded polystyrene is weak and semi-flexible. In small cross-sections, expanded polystyrene patterns can easily be bent or broken by hand. Extreme care is required in all phases of the EPC process to avoid distortion and damage to patterns because any disfigurement ultimately will be reproduced in the casting. The molding operation is particularly critical, because during vibration and compaction of the pattern and sand in a flask, the sand moves, often with considerable force, in currents which are unknown. The pattern is forced to move with the same and is frequently bent or distorted, and sometimes broken, in the molding operation. The final shape of the pattern in the mold after compaction determines the ultimate shape of the casting. Distortion may not occur, or may be unnoticeable, with thick, stocky parts. However, if a part has a large surface and thin cross-section, considerable pattern movement may take place.
Alternatively, a structure or part may be designed with a regular curvature. An example is a tank constructed from plate sections. When such plates are made by casting for specialty applications, the curvature is likely cast as a feature of the plate rather than added by a subsequent forming operation. With EPC, plate section patterns are made flat and bent to the radius of the tank. Pattern distortion due to movement could result in unusable castings.
Those concerned with these and other problems recognize the need for an improved method of casting metal using EPC process patterns.